The transplantation of hematopoietic stem cells and bone marrow belongs to the prior art. This technique includes first a destruction of immunocompetent cells of the recipient by a pretreatment such as by means of chemotherapy or radiotherapy to an extent sufficient to enable growth of the donor cells including stem cells and immunocompetent cells. Generally, stem cells transplantation is used in the case of diseases accompanied by a functional loss of the bone marrow for example in the frame of the therapy of acute leucemia but also for other blood and immune diseases as well as for cancer diseases. This range of diseases to be treated is due to the fact that these are all diseases based on a malfunction in hematopoiesis. Thus, stem cells may be used for non-malignant blood disorders (such as severe aplastic anemia (SAA), aplastic anemia, sickle cell anemia, thalassemia), immune system disorders (multiple sclerosis (MS), rheumatic diseases (CP), scleroderma) as well as for malignant blood disorders (acute and chronic leucemias of myeloid and lymphatic origin). For this purpose, preparations containing stem cells such as bone marrow and blood leucocytes are usually obtained from the donor and administered to the recipient intravenously. A stem cell transplantation of this type can induce tolerance to organs such as heart, lung etc. as well as to blood stem cells of the stem cells donor (1). A disadvantage of this type of stem cell transplantation is, however, that on the one hand the patient transplant may be rejected and on the other hand immunocompetent cells of the transplant may attack and injure cells of the recipient in the course of a graft-versus-host (GVH) disease wherein T cells of the transplant recognize cells of the recipient as non-self. In contrast to the transplantation of solid organs, the immunosuppressive treatment may be discontinued after months or years in the case of a stem cell transplantation if no rejection or GVH reaction occurs. A mutual tolerance is assumed.
To prevent or decrease, respectively, the graft-versus-host reaction, immunocompetent cells, particularly T lymphocytes are frequently removed from the transplant. This, however, quite often results in transplant rejection (2), and in the case of the treatment of leucemia recurrences of the disease increasingly occur. The recurrences in the case of leucemia treatment as well as transplant rejection presumably occur due to a lack of donor T cells in the transplant which would be able to eliminate the T cells remaining after pretreatment of the recipient (3). Such complications do indeed bear a rather high risk even in the case of HLA identity (identity of tissue features) between donor and recipient and a very high risk for HLA differences.
Stem cell transplantation for the induction of a tolerance to transplants or for maintaining the tolerance was initially done based on the consideration that the patient's immune system will be replaced by the immune system of the stem cell donor which does not attack the transplanted organ. Therefore, as soon as an organism has accepted foreign stem cells this organism also will take other organs (4). Since, however, stem cell transplantation across HLA differences bear a high risk this method for the induction of tolerance to transplanted organs is not useful at the moment.
It has been demonstrated in dogs that in special donor-recipient combinations a strong graft-versus-host reaction can be avoided by T cell depletion of the bone marrow. In combinations of DLA homozygous donors and DLA heterozygous recipients the reaction in host-versus-graft direction is weak compared to the GVH direction, a complete chimerism is obtained with sustained tolerance across differences in DLA haplotype (5). Strong DLA differences, however, of the heterozygous donor almost always result in rejection and do not allow for mutual tolerance.
There have already been various attempts and possibilities for the prevention or reduction, respectively, of a rejection due to HLA differences.
For example it is known that the use of high amounts of CD34-positive blood stem cells may induce tolerance in adults (6) and children (7). In the case of adult donors and small recipients as often encountered in parent-to-children transplantations blood stem cells may be obtained in sufficient amounts. If the size relations are more unfavorable it will be problematic to obtains blood stem cells in sufficient amounts.
As another method for the induction of tolerance across HLA differences there has been suggested bone marrow transplantation together with a combination of cyclophosphamide, thymus irradiation and anti-thymocyte globulin (ATG) (8). This procedure has been successful for patients with highly malignant lymphoma but these patients are already immunosuppressed due to their diseases and the preceding chemotherapy. It can be assumed, however, that this type of treatment would not be sufficient for patients with a weaker previous immunosuppression. A further disadvantage of this method is an increased susceptibility of the organism to infections as with all merely unspecific immunosuppressive agents.
Another method is the use of CTLA4, a ligand of the B7.1 co-stimulatory molecule. Among other properties, CTLA4 limits T cell activation on antigen stimulation. The expression of the co-stimulatory molecule B7.1 on antigen-presenting cells is necessary for T cell activation. In the above-mentioned method, CTLA4-IgG fusion protein is added to the bone marrow prior to transplantation where it blocks the B7.1 co-stimulatory molecule, and thus the recipient T cells lack the second signal important for activation. The T cells are unable to react (‘anergy’) and their capability to perform an acute graft-versus-host reaction is decreased (9). This method has been tested in twelve patients, four patients of which were in a group with good risk and eight patients in a group with increased risk. “Good risk” refers to young patients, little therapy attempts, a low amount of leucemic cells and outbreak of the diseases not long ago while these conditions do not apply in the case of a “high risk”. In this method, however, the irradiated blood cells of the recipient must be treated over an incubation period of 36 hours. Besides an increased risk of infection, an increased risk of tolerance induction can be expected.